Given the prevailing progressive fiber optic transmission system upgrade approach, high data rate PM (i.e. 100 Gb/s, 200 Gb/s, etc.) channels must coexist with legacy on-off keying (OOK) neighboring channels in the links designed for such legacy OOK signals. However, numerous field trials and laboratory experiments have revealed a significant nonlinear cross talk penalty in a coherent 100 Gb/s channel, for example, due to XPolM, or nonlinear polarization cross talk, and XPM caused by 10 Gb/s OOK neighboring channels, for example. This nonlinear cross talk penalty reduces the unregenerated reach of high data rate PM signals by several dBs on dispersion compensated links, including non-dispersion shifted fiber (NDSF) and non-zero dispersion shifted fiber (NZDSF), for example. This occurs on most legacy terrestrial and submarine links.
Existing state-of-the-art approaches for dealing with nonlinear XPolM induced noise and nonlinear XPM induced noise can be divided into four general categories: (1) coherent polarization division multiplexed (PDM) systems relying on high speed analog to digital converter/digital signal processing (ADC/DSP) in order to track the polarization variations of the signals due to XPolM and the phase variations of the signals due to XPM; (2) the use of pilot tones to compensate for the inter-channel nonlinear penalties; (3) the use of feed forward DSP and train bit sequences to compensate for XPolM; and (4) the use of DSP based nonlinear back propagation to compensate for intra-channel (i.e. inside a single channel) nonlinear systematic distortions. The drawbacks of these approaches are as follows.
The speed of polarization and phase tracking in conventional coherent PDM systems, relying on high speed ADC/DSP, is typically limited to less than 1 MHz due to the data processing speed and data acquisition scheme. As a consequence, polarization variations of the signals due to XPolM and phase variations of the signals due to XPM, occurring at the speed of ˜1 GHz due to the 10 Gb/s neighboring channels, are far beyond the capabilities of polarization and phase tracking in conventional coherent PDM systems.
In the pilot based approaches that have been proposed for orthogonal polarization detection in PDM systems, the bandwidth of the low pass filters (LPFs) that filter the pilots have been restricted to not more than 100 MHz, to minimize the amplified spontaneous emission (ASE) noise impact. As later analysis has shown, the overwhelming amount of the nonlinear noise due to the nonlinear XPM and XPolM is located in the 1 GHz to 3 GHz spectral range. By this reasoning, the orthogonal polarization approach with pilot filtering within less than 100 MHz bandwidth proposed is only capable of improving the quality (Q) factor by about 0.5 dB. One pilot based nonlinear compensation approach proposed allows for a 2.4 dB Q factor improvement in a single polarization wavelength division multiplexing (WDM) transmission. However, this approach is only capable of compensating the XPM and self phase modulation (SPM), and it does not allow for the compensation of the nonlinear cross polarization noise due to XPolM in PDM systems. Another pilot aided approach proposed has been demonstrated to a yield a fairly limited (i.e. less than 0.6 dB) Q factor improvement in a coherent PDM system due to partial XPM compensation. Numerous alternative pilot aided approaches for XPM compensation also yield limited Q factor improvement, within 1 dB. Note that these approaches do not compensate for XPolM. A further pilot aided approach proposed allows only for a modest 0.5 dB increase in tolerable launch power. Likewise, a pilot tone based approach proposed allows for up to 0.5 dB improvement in the nonlinear tolerance. A still further pilot aided approach allows for a 1 dB increase of the launch power, resulting in a modest 9% increase in a maximum system reach.
The methods that use feed forward DSP and train bit sequences to compensate for XPolM also yield a limited (i.e. 1 dB) Q factor improvement. The reason for this is twofold. First, the train sequences are limited to about 0.1 microseconds, which cuts off the polarization variations beyond 10 MHz. Second, the nonlinear signal phase variations due to XPM remain uncompensated for in this approach.
Finally, digital back propagation that is applied within a single channel bandwidth only produces very limited benefit (i.e. under 1 dB) in a WDM environment, as it does not compensate for the inter-channel XPM and XPolM. Multichannel bandwidth digital back propagation is able to provide much higher gains, but is not implementable in any realistic term DSP technology.